diff --git a/src/main/java/com/xliic/openapi/bundler/Resolver.java b/src/main/java/com/xliic/openapi/bundler/Resolver.java
index c78fc08..6e47215 100644
--- a/src/main/java/com/xliic/openapi/bundler/Resolver.java
+++ b/src/main/java/com/xliic/openapi/bundler/Resolver.java
@@ -7,7 +7,7 @@
 
 import java.net.URI;
 import java.net.URISyntaxException;
-import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.Iterator;
 
 import com.fasterxml.jackson.databind.JsonNode;
@@ -30,10 +30,10 @@ public static boolean isExternalRef(final JsonNode node) {
     }
 
     public static void resolveReference(Parser parser, Reference reference) {
-        resolveReference(parser, reference, new ArrayList<URI>());
+        resolveReference(parser, reference, new HashSet<>());
     }
 
-    public static void resolveReference(Parser parser, Reference reference, ArrayList<URI> visited) {
+    public static void resolveReference(Parser parser, Reference reference, HashSet<URI> visited) {
 
         URI ref;
         try {
diff --git a/src/test/java/com/xliic/openapi/bundler/BundlerTest.java b/src/test/java/com/xliic/openapi/bundler/BundlerTest.java
index 3ea375d..f330333 100644
--- a/src/test/java/com/xliic/openapi/bundler/BundlerTest.java
+++ b/src/test/java/com/xliic/openapi/bundler/BundlerTest.java
@@ -225,11 +225,11 @@ void testSwaggerRemapEmpty() throws JsonProcessingException, IOException, URISyn
             BundlingException, WorkspaceException {
         // make sure that components are mapped to
         // nodes under /definitions and /components
-        // even if the main files dont' have these nodes
+        // even if the main files don't have these nodes
         BundledJsonNode bundled20 = bundle("minimal", "swagger20.yaml");
-        assertNotNull(bundled20.at("/definitions/FooSchema/type"));
+        assertEquals("object", bundled20.at("/definitions/definitions.yaml-definitions-FooSchema/type").textValue());
 
         BundledJsonNode bundled30 = bundle("minimal", "openapi30.yaml");
-        assertNotNull(bundled30.at("/components/schemas/FooSchema/type"));
+        assertEquals("object", bundled30.at("/components/schemas/schemas.yaml-components-schemas-FooSchema/type").textValue());
     }
 }
\ No newline at end of file